The work of Louis Pasteur

Everything follows on logically in the work of Louis Pasteur, from his studies on crystallography to his discovery of virus-vaccinations and the rabies prophylaxis.

The origin of Louis Pasteur’s work is his reading of a paper by a German physician, Mitscherlich. This latter had noticed that two chemical compounds, sodium ammonium tartrate and sodium ammonium paratartrate, have the same chemical composition and crystalline form, but yet have a different action on polarized light. Pasteur will try to discover the cause of this property.

First period :
From 1847 to 1862 (i.e., 25 to 40 years old)

Pasteur is mainly a physicist and a chemist.

1847: work on molecular asymmetry

After several years of research in which he associates crystallography, chemistry and optics, Pasteur shows that the external shape of a crystal, its molecular composition and its action on polarized light are all linked. Polarized light is rotated by asymmetric crystals, not by crystals that have a plane of symmetry.

He states this fundamental law: “only products originating under the influence of life are asymmetrical, because the cosmic [i.e. generative, life] forces that preside over their formation are themselves asymmetrical”.

Asymmetry differentiates the organic world and the mineral world. Pasteur’s work was the origin of a new science, stereochemistry (or spatial chemistry).

This triggers the development of chemical synthesis.

1855-1857: work on fermentation

Pasteur discovered molecular asymmetry by examining paratartrate crystals. When examining a solution of paratartaric acid, he noticed that this acid had fermented and dissociated under the effect of mould: only the left-handed tartaric acid could be found in the fermented liquid, while the right-handed tartaric acid had decomposed. Thus, the substance that was inactive under polarized light (paratartaric acid) had become active (left-handed tartaric acid) under the influence of fermentation.

Therefore, since every active substance originates from life, fermentation must be the work of life, instead of being the work of death as the chemist Liebig believed. Thus, only life can generate substances that are active on polarized light. This is the first step that will lead him from molecular asymmetry through fermentation to contagious diseases, via a logical sequence in his studies.

1857-1862

He studies lactic and alcoholic fermentation and shows that:

♦ all fermentation is due to the presence of a microorganism;
♦ for a given fermentation, there is a specific corresponding ferment.

In order to study a fermentation, he also shows it is necessary to:

♦ prepare a sterile culture medium that is suitable for the ferment;
♦ seed this medium with a trace of the ferment in a pure state.

This is the origin of all microbiological techniques

Pasteur worked as a chemist-biologist to study the mechanisms of fermentation. This led him to confirm the role and the specific nature of microorganisms.

Second period :
From 1862 to 1877 (i.e., 40 to 55 years old)

During this period, Pasteur becomes a biologist. He builds up the germ theory and annihilates the spontaneous generation theory.

Following his first discoveries, he asks questions about the origin of the microorganisms that are the agents of fermentation. Do they originate from germs similar to themselves or do they appear spontaneously in germ-favourable media? This was the spontaneous generation controversy. After some unforgettable struggles against his opponents (Pouchet), Pasteur could claim in his 1862 paper, on the basis of various experiments, that:

even the most putrescible liquids remain unadulterated if care is taken to keep them away from these germs.

“Spontaneous generation is a chimera” (Pasteur)

He wonders how fermentations operate and how ferments work. By studying butyric fermentation, he discovers a new class of living beings, able to live in air-free conditions. He suggests the term of “anaerobic” for ferments that have the property of living in air-free conditions, the term “aerobic” designating microorganisms that require the presence of free oxygen in order to develop.

Fermentation is a consequence of air-free life

The work on fermentation leads Pasteur to apply the microbiological method to industry and agriculture.

He studies the formation of vinegar and the transformation of alcohol into acetic acid by a microorganism, Mycoderma aceti, which fixes oxygen from the air onto the alcohol. He shows vinegar makers how to obtain vinegar of a consistent quality.

Wine diseases

By studying the parasitic ferments of this beverage, Pasteur shows that each disease is due to a specific ferment. With a special heating at 55º C, it is possible to prevent diseases from affecting wines. This method, applied to all fermentable liquids, is known worldwide by the name of “Pasteurisation”.

Beer

Microorganisms brought in by airborne dust produce alterations in beer. Pasteur teaches brewers to protect the wort from dirt and heat beer to 55ºC to prevent diseases.

Silkworm diseases

In 1865, a disease devastates the silkworm industry, not only in France but also in Italy, Austria and Asia Minor: the silkworm nosema disease.
Pasteur notices, under the microscope, that worms suffering from this disease show shiny corpuscles and that these corpuscles are responsible for the disease.
He shows that nosema disease is hereditary and contagious.
He discovers another disease, known as flacherie, thus highlighting the notion of a specific “terrain” favourable for the outbreak of a disease.
He practically eliminates the disease by inventing cellular egg production.
Pasteur’s work has a huge importance: for the first time, heredity and contagion problems are solved, and prophylaxis rules are established. This is the prelude to his studies on contagious diseases and germ theory.

Third period :
From 1877 to 1887 (i.e., 55 to 65 years old)

In this decade, Pasteur uses microbiology for medicine and surgery.

1877 : study of infectious diseases

discovery of the cause of furuncles and osteomyelitis: staphylococcus

discovery of the puerperal infection microbe: streptococcus

discovery of pneumococcus

1880 : methods of attenuating microbe virulence

chicken cholera (by aging in contact with the air oxygen)

sheep anthrax (by culture of anthrax bacillus at 43ºC, attenuated by air oxygen)

1881 : vaccinations

cholera

anthrax

swine erysipelas

By applying his method to :

the study of infectious diseases (microbial agents)

their prevention (asepsis)

their prophylaxis by immunization (vaccination)

Pasteur founds the science of immunology.

1880-1885 : rabies

Pasteur masters his experimental method. He studies rabies. He tries to isolate the germ but cannot find it. Rabies is a disease affecting the nervous system. He grows an invisible “micro-organism” on rabbit marrow and attenuates its virulence.

He applies this method of attenuation of virulent marrows to human beings for the first time on July 6th, 1885, when he treats Joseph Meister.